The main objective of this study is to further define tinnitus-like changes in the cochlear nucleus following exposure to intense sound. This study builds on the recent discovery that intense sound exposure at levels sufficient to induce tinnitus in humans leads to a dramatic increase in spontaneous neural activity in the dorsal cochlear nucleus of the hamster. The specific aims of this project are as follows: 1) To further define the time course of these increases, spontaneous activity will be recorded and systematically mapped as a function of location along the frequency axis for each of 5 postexposure recovery times from 2 days to 1 year. The locations of abnormal spontaneous activity will be marked iontophoretically with horseradish peroxidase. 2) The cochlear nuclei will then be analyzed to determine whether increases in spontaneous activity are accompanied by significant chemical changes. The concentrations of selected neurotransmitter candidates, neurotransmitter- associated enzymes, and neurotransmitter receptors will be assayed using high performance liquid chromatography and autoradiography. Changes in key neurotransmitter candidates and associated enzymes will be mapped histochemically along the tonotopic axis. Chemically altered areas will be identified and correlated with regions showing high spontaneous activity by reference to marks of horseradish peroxidase. 3) Electrophysiological recordings of spontaneous neural discharge rates, amplitudes, and temporal structure will be conducted at the single unit level to identify the neural correlates of tone-induced increases in spontaneous activity. 4) Psychoacoustic testing procedures will be employed to determine whether hamsters exposed to intense sound experience tinnitus, and if so, whether the tinnitus has a definable pitch. Findings from these experiments are expected to contribute to an understanding of central mechanisms of noise-induced tinnitus and pave the way toward the development of anti-tinnitus therapies.